Fuel cell system and control method of fuel cell system

ABSTRACT

Provided is a fuel cell system including: a fuel cell that supplies electricity to a load; a fuel cell converter that is connected between the fuel cell and the load and boosts a voltage output from the fuel cell; and a control unit that causes the fuel cell converter to perform a voltage boosting action and controls output electricity to the load. Upon detecting a voltage boosting disabling failure that is a failure in which the fuel cell converter is unable to perform the voltage boosting action and able to pass a current, the control unit stops the voltage boosting action of the fuel cell converter and passes a current through the fuel cell converter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2017-247795 filed on Dec. 25, 2017, which is incorporated herein byreference in its entirety including the specification, drawings andabstract.

BACKGROUND 1. Technical Field

The present disclosure relates to a fuel cell system and a controlmethod of the fuel cell system.

2. Description of Related Art

In fuel cell systems, a fuel cell converter disposed between a fuel celland a load may be shut off upon failure of the fuel cell converter fromthe viewpoint of fail-safety. Japanese Patent Application PublicationNo. 2011-228294 describes a fuel cell system in which, upon failure of afuel cell converter, the fuel cell converter is shut off and electricityis supplied from a secondary battery to a drive motor that is a load ofa fuel cell electric vehicle.

SUMMARY

A fuel cell system configured to supply electricity only from asecondary battery to a load, such as a drive motor, when a fuel cellconverter has failed during travel of the fuel cell electric vehicle isdisadvantageous in that the distance the vehicle can travel is extremelyshort. Therefore, a technique has been sought that can prevent thedistance the vehicle can travel from shortening upon failure of a fuelcell converter.

(1) A first aspect of the present disclosure relates to a fuel cellsystem including: a fuel cell that supplies electricity to a load; afuel cell converter that is connected between the fuel cell and the loadand boosts a voltage output from the fuel cell; and a control unit thatcauses the fuel cell converter to perform a voltage boosting action andcontrols output electricity to the load. Upon detecting a voltageboosting-disabling failure that is a failure in which the fuel cellconverter is unable to perform the voltage boosting action and able topass a current, the control unit stops the voltage boosting action ofthe fuel cell converter and passes a current generated by the fuel cellthrough the fuel cell converter. In the fuel cell system of this aspect,when the voltage boosting-disabling failure in which the fuel cellconverter is unable to perform the voltage boosting action and able topass a current is detected, the voltage boosting action of the fuel cellconverter is stopped and a current is passed through the fuel cellconverter. Thus, electricity can be continuously supplied from the fuelcell to the load. Therefore, shortening of the distance the fuel cellelectric vehicle can travel can be prevented compared with ifelectricity is supplied only from the secondary battery.

(2) The above fuel cell system may further include: a secondary batterythat supplies electricity to the load; a secondary battery converterthat is connected between the secondary battery and the load, has anoutput terminal electrically connected to an output terminal of the fuelcell converter, and converts an output voltage of the secondary battery;a first voltage sensor that detects a voltage value on a secondary sideof the fuel cell converter; and a second voltage sensor that detects avoltage value on a secondary side of the secondary battery converter.The voltage boosting-disabling failure may include a failure of thefirst voltage sensor. When the voltage boosting-disabling failure is thefailure of the first voltage sensor, the control unit may control theoutput electricity using a detection value of the second voltage sensoras the voltage value on the secondary side of the fuel cell converter.In this fuel cell system, when the voltage boosting-disabling failure isthe failure of the first voltage sensor, the detection value of thesecond voltage sensor is used as the voltage value on the secondary sideof the fuel cell converter. Thus, the output electricity of the fuelcell system can be continuously controlled.

(3) The above fuel cell system may further include: a first voltagesensor that detects a voltage value on a secondary side of the fuel cellconverter; and a fuel cell voltage sensor that detects a voltage valueof the fuel cell. The voltage boosting-disabling failure may include afailure of the first voltage sensor. When the voltage boosting-disablingfailure is the failure of the first voltage sensor, the control unit maycontrol the output electricity using a detection value of the fuel cellvoltage sensor as the voltage value on the secondary side of the fuelwell converter. In the fuel cell system of this aspect, when the voltageboosting-disabling failure is the failure of the first voltage sensor,the detection value of the fuel cell voltage sensor is used as thevoltage value on the secondary side of the fuel cell converter. Thus,the output electricity of the fuel cell system can be continuouslycontrolled.

(4) The above fuel cell system may further include: a secondary batterythat supplies electricity to the load; a first current sensor thatdetects a current value of the fuel cell converter; and a second currentsensor that detects a current value of the secondary battery. Thevoltage boosting-disabling failure may include a failure of the firstcurrent sensor. When the voltage boosting-disabling failure is thefailure of the first current sensor, the control unit may estimate thecurrent value of the fuel cell converter by using a detection value ofthe second current sensor and total electricity supplied by the fuelcell system, and control the output electricity rising the estimatedcurrent value as the current value of the fuel cell converter. In thisfuel cell system, when the voltage boosting-disabling failure is thefailure of the first current sensor, the current value of the fuel cellconverter is estimated by using the detection value of the secondcurrent sensor and the total electricity supplied by the fuel cellsystem, and the estimated current value is used as the current value ofthe fuel cell converter. Thus, the output electricity of the fuel cellsystem can be continuously controlled.

A second aspect of the present disclosure relates to a control method ofa fuel cell system having a fuel cell converter that is connectedbetween fuel cell and a load and boosts an output voltage of the fuelcell. This control method includes, upon detecting a voltageboosting-disabling failure that is a failure in which the fuel cellconverter is unable to perform a voltage boosting action and able topass a current, stopping the voltage boosting action of the fuel cellconverter and passing a current generated by the fuel cell through thefuel cell converter.

The present disclosure can also be implemented in various other formsthan the fuel cell system. For example, the disclosure can beimplemented in the form of a vehicle including the fuel cell system.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a schematic diagram showing an electric system of a fuel cellsystem; and

FIG. 2 is a flowchart showing the sequence of a failure-response controlprocess.

DETAILED DESCRIPTION OF EMBODIMENTS A. First Embodiment A-1.Configuration of Fuel Cell System

FIG. 1 is a schematic diagram showing an electric system of a fuel cellsystem as an embodiment of the present disclosure. A fuel cell system 10is installed in a fuel cell electric vehicle (not shown) as a systemthat supplies electricity for driving. The fuel cell system 10 supplieselectricity to a load 70 to be described later. The load 70 includes adrive motor 72 and an air compressor 74.

The fuel cell system 10 includes a fuel cell 20, a fuel cell converter(hereinafter also referred to as an FDC) 30, a fuel cell relay circuit29, a secondary battery 40, a secondary battery relay circuit 49, issecondary battery converter (hereinafter also referred to as a BDC) 50,an inverter 60, auxiliaries 80, and a control unit 90.

A source of electricity for the fuel cell system 10, the fuel cell 20 isa so-called polymer electrolyte fuel cell, and generates electricity byreceiving a supply of a fuel gas and an oxidant gas. Instead of being apolymer electrolyte fuel cell, the fuel cell 20 may be another arbitrarytype of fuel cell, such as a solid oxide fuel cell. The fuel cell 20 hasa stack structure in which a plurality of single cells (not shown) isstacked.

A cell monitor 21 is connected to the fuel cell 20. The cell monitor 21detects a cell voltage of each single cell of the fuel cell 20. Thetotal value of the cell voltages is equivalent to the voltage of thefuel cell 20. The cell monitor 21 outputs detection values of the cellvoltages and the voltage of the fuel cell 20 to the control unit 90. Athird voltage sensor 22 is connected to an output terminal of the fuelcell 20. The third voltage sensor 22 detects the voltage of the fuelcell 20. In other words, the third voltage sensor 22 detects the voltagebefore being boosted by the FDC 30.

The FDC 30 boosts the output voltage of the fuel cell 20 in accordancewith a command from the control unit 90. Specifically, the FDC 30 booststhe voltage supplied through a primary-side line 33 and supplies theboosted voltage to a secondary-side line 34. A primary side means a sideto which electricity is supplied, i.e., an input side, and a secondaryside means a side from which electricity is supplied, i.e. an outputside. The boosted voltage is supplied to the inverter 60 through thefuel cell relay circuit 29. The FDC 30 adjusts the output electricity ofthe fuel cell 20 by adjusting a current passing through the FDC 30.

The FDC 30 is configured as a four-phase bridge converter. The FDC 30has intelligent power modules (IPMs) 31 of four phases of a U-phase,V-phase, W-phase and X-phase, a first current sensor 36, a capacitor Ca,and a first voltage sensor 37. The IPMs 31 are connected in parallel toone another and have the same configuration. In FIG. 1, the IPM 31 ofone of the four phases is shown as a representative.

Each IPM 31 is a circuit module incorporating a plurality ofsemiconductor elements, and has a reactor La, a backflow preventiondiode DX, a switching, element Sa, a first temperature sensor 38, and athird current sensor 39. The reactor La is connected between a positiveterminal of the fuel cell 20 and an anode terminal of the backflowprevention diode DX. The reactor La can store electricity and dischargestored electricity. The backflow prevention diode DX prevents backflowof a current from the secondary side of the FDC 30 toward the primaryside. The switching element Sa is connected between a low potential-sideline and a point of connection between the reactor La and the backflowprevention diode DX. The switching element Sa is an insulated gatebipolar transistor (IGBT). Instead of being an IGBT, the switchingelement Sa may be another arbitrary type of switching element, such as abipolar transistor or an MOSFET. The first temperature sensor 38 detectsthe temperature of the IPM 31 and outputs the detection value to thecontrol unit 90. The third current sensor 39 detects a current passingthrough the IPM 31 and outputs the detection value to the control unit90.

The first current sensor 36 disposed on a high potential-side, anddetects a current passing through the FDC 30 and outputs the detectionvalue to the control unit 90. The capacitor Ca is disposed on thesecondary side of the FDC 30. The first voltage sensor 37 detects avoltage value on the secondary side of the FDC 30, i.e., the value thevoltage having been boosted by the FDC 30, and outputs the detectionvalue to the control unit 90.

The fuel cell relay circuit 29 is disposed between the FDC 30 and theinverter 60. The fuel cell relay circuit 29 switches betweenelectrically connecting and disconnecting the FDC 30 and the inverter 60to and from each other in accordance with a command from the controlunit 90. The fuel cell relay circuit 29 has an FC first relay FCRB, anFC second relay FCRG, an FC pre-charge relay FCRP, and a limitingresistor R1. The FC first relay FCRB is disposed on the highpotential-side line. The FC second relay FCRG is disposed on the lowpotential-side line. The FC pre-charge relay FCRP is connected inparallel to the EC second relay FCRG. The limiting resistor R1 isconnected in series with the FC pre-charge relay FCRP. The FC pre-chargerelay FCRP and the limiting resistor R1 suppress contact welding of theFC second relay FCRG. The fuel cell relay circuit 29 may be disposedbetween the fuel cell 20 and the FDC 30.

The secondary battery 40 functions as a source of electricity for thefuel cell system 10 along with the fuel cell 20, and supplieselectricity to the load 70 and the auxiliaries 80. In this embodiment,the secondary battery 40 is a chargeable and dischargeable lithium-ionbattery. Instead of being a lithium-ion battery, the secondary battery40 may be another arbitrary type of secondary battery, such as anickel-metal hydride battery. The secondary battery 40 is connected inparallel to the fuel cell 20 relative to the load 70. The secondarybattery 40 is charged with regenerated electricity generated by thedrive motor 72.

A second current sensor 41 and a fourth voltage sensor 44 areelectrically connected to an output terminal of the secondary battery40. The second current sensor 41 detects a current value of thesecondary battery 40 and outputs the detection value to the control unit90. The fourth voltage sensor 44 detects an output voltage of thesecondary battery 40 and outputs the detection value to the control unit90. A second temperature sensor 42 and an SOC detection unit 43 are alsoconnected to the secondary battery 40. The second temperature sensor 42detects the temperature of the secondary battery 40 and outputs thedetection value to the control unit 90. The SOC detection unit 43detects the state of charge (SOC) indicating the amount of electricitystored in the secondary battery 40, and outputs the detection value tothe control unit 90.

The secondary battery relay circuit 49 is disposed between the secondarybattery 40 and the BDC 50. The secondary battery relay circuit 49switches between electrically connecting and disconnecting the secondarybattery 40 and the BDC 50 to and from each other in accordance with acommand from the control unit 90. The secondary battery relay circuit 49has a secondary battery first relay SMRB, a secondary battery secondrelay SMRG, a secondary battery pre-charge relay SMRP and a limitingresistor R2. The secondary battery first relay SMRB is disposed on ahigh potential-side line. The secondary battery second relay SMRG isdisposed on a low potential-side line. The secondary battery pre-chargerelay SMRP is connected in parallel to the secondary battery secondrelay SMRG. The limiting resistor R2 is connected in series with thesecondary battery pre-charge relay SMRP. The secondary batterypre-charge relay SMRP and the limiting resistor R2 suppress contactwelding of the secondary battery second relay SMRG.

The BDC 50 converts the output voltage of the secondary battery 40 inaccordance with a command from the control unit 90. Specifically, theBDC 50 boosts a voltage supplied through a primary-side line 53 andsupplies the boosted voltage to a secondary-side line 54. The boostedvoltage is supplied to the inverter 60. The BDC 50 controls a portion ofelectricity of the fuel cell system 10 that is output from the secondarybattery 40, by adjusting a current passing through the BDC 50. An outputterminal of the BDC 50 and an output terminal of the FDC 30 areelectrically connected to each other.

In this embodiment, the BDC 50 is a non-isolated DC-DC converter. TheBDC 50 may instead be a bidirectional DC-DC converter that can step downa voltage input through the secondary-side line 54 and supply thestepped-down voltage to the secondary battery 40 through theprimary-side line 53. The BDC 50 may constitute a power control unit(PCU) along with the inverter 60 and a step-down converter 89.

The BDC 50 has an upper arm 56, a lower arm 58, a reactor L1, capacitorsC1, C2, and a fifth voltage sensor 45. The upper arm 56 includes a firstswitching element S1 and a first diode D1. The first switching elementS1 is an IGBT. Instead of being an IGBT, the first switching element S1may be another arbitrary type of switching element, such as a bipolartransistor or an MOSFET. The first diode D1 is connected in antiparallelto the first switching element S1. The lower arm 58 includes a secondswitching element S2 and a second diode D2, and has the sameconfiguration as the upper arm 56. The upper arm 56 and the lower arm 58are connected in series with each other. The reactor L1 is connected toa point of connection between the upper arm 56 and the lower arm 58. Thecapacitor C1 is disposed on the primary-side line 53, and the capacitorC2 is disposed on the secondary-side line 54. The fifth voltage sensor45 is disposed on the primary-side line 53 and detects a voltage on theprimary side of the BDC 50.

The inverter 60 converts direct-current electricity supplied from thefuel cell 20 and the secondary battery 40 into three-phasealternating-current electricity. The inverter 60 supplies the convertedelectricity to the load 70. A second voltage sensor 25 and a capacitorC3 are disposed at an input terminal of the inverter 60. The secondvoltage sensor 25 detects the value of a voltage input into the inverter60, i.e., the value of the voltage on the secondary side of the BDC 50,and outputs the detection value to the control unit 90. The capacitor C3suppresses fluctuations in the voltage input into the inverter 60.

The load 70 includes the drive motor 72 and the air compressor 74. Thedrive motor 72 drives wheels (not shown) of the fuel cell electricvehicle. The air compressor 74 pumps an oxidant gas to the fuel cell 20.Output torques of synchronous motors of the drive motor 72 and the aircompressor 74 are controlled as the inverter 60 is controlled by thecontrol unit 90. The drive motor 72 is not a component of the fuel cellsystem 10, but the air compressor 74 is a component of the fuel cellsystem 10. For the convenience of description, the load 70 in thisembodiment will be described as not being a component of the fuel cellsystem 10.

The auxiliaries 80 are connected to the primary-side line 53 of the BDC50. The auxiliaries 80 include high-voltage auxiliaries 81 that requirehigh voltages to drive and low-voltage auxiliaries 88 that require lowvoltages to drive.

The high-voltage auxiliaries 81 include a hydrogen pump 82, a coolantpump 83, and a coolant heater 84. The hydrogen pump 82 returns anoff-gas discharged from the fuel cell 20 back to a fuel gas supplypassage. The coolant pump 83 circulates a coolant flowing inside thefuel cell 20. The coolant heater 84 beats the coolant such that waterinside the fuel cell 20 does not freeze. The high-voltage auxiliaries 81may include an air-conditioning device 85 of the filet cell electricvehicle etc. as a device that is not included in the fuel cell system10. An auxiliary inverter 86 is connected to the high-voltageauxiliaries 81. The auxiliary inverter 86 converts direct-currentelectricity into three-phase alternating-current electricity andsupplies the converted electricity to the high-voltage auxiliaries 81.

The low-voltage auxiliaries 88 include a flow control valve that isdisposed in a flow passage through which a reactant gas and the coolantare supplied to or discharged from the fuel cell 20. The low-voltageauxiliaries 88 are supplied with electricity of which the voltage hasbeen lowered by the step-down converter 89 to about 12 V.

The control unit 90 is a microcomputer including a central processingunit (CPU) and a storage device, and is configured as an electroniccontrol unit (ECU). The CPU executes a program stored in advance in thestorage device, and thereby executes control over an electricitygeneration operation of the fuel cell 20, a voltage boosting action ofthe FDC 30, a voltage boosting action of the BDC 50, output electricityto the load 70, and actions of the inverter 60, the auxiliary inverter86, and the step-down converter 89. The output electricity to the load70, i.e., the output electricity of the fuel cell system 10, iscontrolled as follows. The control unit 90 determines proportions ofelectricity to be supplied respectively from the fuel cell 20 and thesecondary battery 40 relative to electricity required by the load 70,and determines a current and an output voltage of the FDC 30 and acurrent and an output voltage of the BDC 50. Then, the control unit 90transmits a command ordering the FDC 30 and the BDC 50 to change theduty ratios of the switching elements Sa, S1, S2. Moreover, the controlunit 90 detects a failure of the FDC 30 and controls the FDC 30according to the type of the failure detected.

In the fuel cell system 10 of this embodiment, a failure-responsecontrol process to be described below is executed, and an operation modeof the fuel cell system 10 is thereby shifted to a mode in which thevoltage boosting action of the FDC 30 is stopped and a current is passedthrough the FDC 30 (hereinafter referred to as a voltage boosting-lessmode), according to the type of the failure of the FDC 30. Thus,electricity is continuously supplied from the fuel cell 20 to preventshortening of the distance the fuel cell electric vehicle can travel.

A-2. Failure-Response Control Process

FIG. 2 is a flowchart showing the sequence of the failure-responsecontrol process. The failure-response control process is executed aftera starter switch (not shown) of the fuel cell electric vehicle ispressed and the fuel cell system 10 starts.

The control unit 90 determines whether there is any failure occurring inthe FDC 30 (step S210).

Failures of the FDC 30 include the following (a) to (c):

(a) a failure in which the FDC 30 can perform the voltage booting actionand can pass a current;

(b) a failure in which the FDC 30 cannot perform the voltage boostingaction but can pass a current (hereinafter also referred to as a voltageboosting-disabling failure); and

(c) a failure in which the FDC 30 can neither perform the voltageboosting action nor pass a current.

The failure of (a) means a failure in which the FDC 30 can perform thevoltage boosting action. Examples of the failure of (a) include afailure of the IPM 31 of one or more of the four phases of the FDC 30.As the failure of the IPM 31, for example, a failure of the switchingelement Sa, a failure of the first temperature sensor 38, a failure ofthe third current sensor 39, and short-circuit and disconnection insidethe circuit forming the IPM 31 are assumed.

The failure of (b) means a failure in which the FDC 30 cannot performthe voltage boosting action but electricity can be conducted between theprimary-side line 33 and the secondary-side line 34 of the FDC 30.Examples of the failure of (b) include a failure of the first voltagesensor 37, a failure of the first current sensor 36, and failures of thefirst temperature sensors 38 of all the four phases.

The failure of (c) means a failure in which electricity cannot beconducted between the primary-side line 33 and the secondary-side line34 of the FDC 30. Examples of the failure of (c) include a failure ofthe FDC 30 due to an overcurrent, overvoltage, circuit malfunction, etc.

Failures of the FDC 30 are detected by various methods. For example,short-circuit of the circuit forming the FDC 30 is detected when avoltage in a line reaches a voltage (e.g., 5 V) exceeding apredetermined valid value for the line (e.g., 0.1 V to 4.9 V).Similarly, disconnection of the circuit forming the FDC 30 is detectedwhen a voltage in a line reaches a voltage (e.g., 0 V) lower than thevalid value. For example, an overcurrent is detected when a currentequal to or higher than a predetermined current value flows, and anovervoltage is detected when a voltage equal to or higher than apredetermined voltage value is applied. For example, failures of sensorsare detected by comparing detection values of different sensors by aso-called two value comparison, or comparing a detection value and anestimation value. Failures of sensors may be comprehensively detected bycombining a plurality of two-value comparisons, estimation valuecomparisons, etc.

For example, a comparison between a detection value of the first voltagesensor 37 and a detection value of the second voltage sensor 25 is usedto detect a failure of the first voltage sensor 37. The detection valuesof the first voltage sensor 37 and the second voltage sensor 25 aretheoretically equal. In this embodiment, therefore, a failure of thefirst voltage sensor 37 is detected when the difference between thedetection values of the first voltage sensor 37 and the second voltagesensor 25 falls outside a range of error.

For example, a comparison between a detection value of the first currentsensor 36 and the sum of detection values of the third current sensors39 is used to detect a failure of the first current sensor 36. Thedetection value of the first current sensor 36 and the sum of thecurrent values of the IPMs 31 detected by the third current sensors 39of the respective IPMs 31 are theoretically equal. In this embodiment,therefore, a failure of the first current sensor 36 is detected when thedifference between the detection value of the first current sensor 36and the sum of the detection values of the third current sensors 39falls outside a range of error.

For example, a comparison among detection values of the third currentsensors 39 of the phases is used to detect a failure of the thirdcurrent sensor 39 of each phase. In this embodiment, a failure of thethird current sensor 39 of one phase is detected when the differencebetween the detection value of the third current sensor 39 of that onephase and the detection values of the third current sensors 39 of theother three phases falls outside a range of error.

For example, a comparison between an estimation value and a detectionvalue of a temperature rise is used to detect a failure of the firsttemperature sensor 38 of each phase. In this embodiment, a failure ofthe first temperature sensor 38 is detected, when a temperature rise perunit time is calculated from a detection value of the first temperaturesensor 38 and the difference between this calculated temperature riseand a temperature rise per unit time estimated from an electricity lossdue to heat generation accompanying the voltage boosting action fallsoutside a range of error.

When it is determined in step S210 that there is no failure occurring inthe FDC 30 (step S210: NO), the control unit 90 returns to step S210.

On the other hand, when it is determined in step S210 that there is afailure occurring in the FDC 30 (step S210: YES), the control unit 90identifies the failure that is occurring (step S220).

The control unit 90 determines whether the identified failure of the FDC30 is a failure in which the FDC 30 cannot perform the voltage boostingaction (step S230). In other words, the control unit 90 determineswhether the failure of the FDC 30 is the failure of (b) or the failureof (c).

When it is determined in step S230 that the failure is not a failure inwhich the FDC 30 cannot perform the voltage boosting action, i.e., thefailure is the failure of (a) (step S230: NO), the control unit 90causes the FDC 30 to stop the voltage boosting action in the phasehaving the failure and to perform the voltage boosting action only inthe phases not having a failure (step S235). For example, when the IPM31 of one phase among the IPMs 31 of the four phases fails, the controlunit 90 causes the FDC 30 to stop the voltage boosting action by fixingthe duty ratio of the switching element Sa of the phase having thefailure at zero, and to perform the voltage boosting action in the otherthree phases not having a failure. The output is lower when the voltageboosting action is performed using the IPMs 31 of three phases than whenthe voltage boosting action is performed using the IPMs 31 of all thefour phases.

Since the voltage boosting action of the FDC 30 is stopped in the phasehaving the failure and performed only in the phases not having a failurein step S235, electricity can be continuously supplied from the fuelcell 20 to the load 70. After step S235, the control unit 90 returns tostep S210. The control unit 90 may notify a user of the fuel cellelectric vehicle of the failure of the FDC 30 and prompt the user tobring the vehicle to a service shop etc.

On the other hand, when it is determined in step S230 that the failureis a failure in which the FDC 30 cannot perform the voltage boostingaction (step S230: YES), the control unit 90 determines whether thefailure of the FDC 30 is a failure in which the FDC 30 can pass acurrent (step S240). In other words, the control unit 90 determineswhether the failure of the FDC 30 is the failure of (b), of the failureof (b) and the failure of (c).

When it is determined in step S240 that the failure is not a failure inwhich the FDC 30 can pass a current, i.e., the failure is a failure inwhich the FDC 30 cannot pass a current as in the failure of (e) (stepS240: NO), no current can be passed through the FDC 30. Therefore, thecontrol unit 90 sets the operation mode of the fuel cell system 10 to asecondary battery running mode (step S260).

In the secondary battery running mode, electricity is supplied usingonly the secondary battery 40. The control unit 90 stops the FDC 30,stops electricity generation of the fuel cell 20, and opens the fuelcell relay circuit 29 (step S270). Stopping the FDC 30 means directlyconnecting the primary-side line 33 and the secondary-side line 34 toeach other without the FDC 30 performing the voltage boosting action.Specifically, the duty ratios of the switching elements Sa of the IPMs31 of all the four phases are fixed at zero. Stopping electricitygeneration of the fuel cell 20 means stopping the supply of a fuel gasand an oxidant gas to the fuel cell 20. After step S270, thefailure-response control process is ended.

On the other hand, when it is determined in step S240 that the failureis a failure in which the FDC 30 can pass a current (step S240: YES),the control unit 90 sets the operation mode of the fuel cell system 10to the voltage boosting-less mode (step S250). Specifically, upondetecting the voltage boosting-disabling failure that is a failure inwhich the FDC 30 cannot perform the voltage boosting action but can passa current, the control unit 90 stops the voltage boosting action of theFDC 30 and passes a current through the FDC 30.

In the voltage boosting-less mode, the voltage boosting action of theFDC 30 is stopped and the primary-side line 33 and the secondary-sideline 34 are directly connected to each other to pass a current throughthe FDC 30. Thus, the FDC 30 functions merely as a circuit to pass acurrent. The voltage supplied from the fuel cell 20 is output to theinverter 60 without undergoing the voltage boosting action by the FDC30.

When a failure of the first voltage sensor 37, a failure of the firstcurrent sensor 36, failures of the first temperature sensors 38 of allthe four phases, or the like occurs, continuing the voltage boostingaction of the FDC 30 may result in a significant difference between theactual output and the required output. By comparison, passing a currentthrough the FDC 30 is less likely to cause significant fluctuations inthe output electricity.

As has been described above, the output electricity of the fuel cellsystem 10 is controlled through determination of the current and theoutput voltage of the FDC 30 and the current and the output voltage ofthe BDC 50. Thus, controlling the output electricity requires the outputvoltage of the FDC 30 and the current value of the FDC 30. As mentionedabove, the output voltage of the FDC 30 is detected by the first voltagesensor 37. Therefore, when the first voltage sensor 37 has failed, it isdesirable to obtain the output voltage of the FDC 30 by another method.The current value of the FDC 30 is detected by the first current sensor36. Therefore, when the first current sensor 36 has failed, it isdesirable to obtain the current value of the FDC 30 by another method.

The control unit 90 determines whether the failure of the FDC 30 is afailure of the first voltage sensor 37 (step S252). When it isdetermined that the failure of the FDC 30 is a failure of the firstvoltage sensor 37 (step S252: YES), the control unit 90 switches theoutput voltage value of the FDC 30 to be used for controlling the outputelectricity (step S254).

In step S254, the voltage value to be used as the output voltage valueof the FDC 30 is switched from the voltage value detected by the firstvoltage sensor 37 to the voltage value detected by the second voltagesensor 25. This measure takes advantage of the fact that the outputvoltage value of the FDC 30 detected by the first voltage sensor 37 andthe voltage value detected by the second voltage sensor 25 aretheoretically equal.

After step S254, the failure-response control process is ended. Thecontrol unit 90 controls the output electricity of the fuel cell system10 by using the voltage value having been switched in step S254.

On the other hand, when it is determined in step S252 that the failureof the FDC 30 is not a failure of the first voltage sensor 37 (stepS252: NO), the control unit 90 determines whether the failure of the FDC30 is a failure of the first current sensor 36 (step S256). When it isdetermined that the failure of the FDC 30 is not a failure of the firstcurrent sensor 36 (step S256: NO), the failure-response control processis ended. On the other hand, when it is determined that the failure ofthe FDC 30 is a failure of the first current sensor 36 (step S256: YES),the control unit 90 switches the current value of the FDC 30 to be usedfor controlling the output electricity (step S258).

In step S258, the control unit 90 estimates the current value of the FDC30 by using the total electricity supplied by the fuel cell system 10and the current value of the secondary battery 40 detected by the secondcurrent sensor 41. Then, the control unit 90 switches the current valueto be used as the current value of the FDC 30 from the detection valueof the first current sensor 36 to this estimation value.

The total electricity supplied by the fuel cell system 10 is equal tothe sum of electricity consumed by the load 70 and electricity consumedby the auxiliaries 80. The electricity consumed by the load 70 includingthe drive motor 72 and the air compressor 74 can be calculated, forexample, from a current value detected by a current sensor (not shown)disposed at the inverter 60 and a voltage input into the inverter 60. Ofthe electricity consumed by the auxiliaries 80, electricity consumed bythe high-voltage auxiliaries 81 can be calculated, for example, from acurrent value detected by a current sensor (not shown) disposed at theauxiliary inverter 86 and a voltage detected by the fifth voltage sensor45. Of the electricity consumed by the auxiliaries 80, electricityconsumed by the low-voltage auxiliaries 88 accounts for a smallproportion of the entire consumed electricity, and therefore may beregarded as a constant. For example, the current flowing through thelow-voltage auxiliaries 88 may be assumed to be 100 A, and theelectricity consumed by the low-voltage auxiliaries 88 may be regardedas 1.2 kW from the voltage value of 12 V to which the voltage has beenstepped down by the step-down converter 89.

The total electricity supplied by the fuel cell system 10 comes fromelectricity supplied from the fuel cell 20 and the secondary battery 40.It is therefore possible to estimate the electricity supplied from thefuel cell 20 and estimate the current value of the FDC 30, by obtainingthe electricity supplied from the secondary battery 40 based on thecurrent value of the secondary battery 40 detected by the second currentsensor 41 and on the voltage value of the secondary battery 40 detectedby the fourth voltage sensor 44, and then subtracting this electricityfrom the total electricity supplied by the fuel cell system 10. When theSOC is equal to or higher than a predetermined value, the voltage valueof the secondary battery 40 may be a fixed value instead of the valuedetected by the fourth voltage sensor 44.

After step S258, the failure-response control process is evaded. Thecontrol unit 90 controls the output electricity of the fuel cell system10 by using the current value estimated in step S258. After thefailure-response control process is ended, the control unit 90 maynotify the user of the fuel cell electric vehicle of the failure of theFDC 30 and prompt the user to bring the vehicle to a service shop etc.

In the fuel cell system 10 of the embodiment having been describedabove, when the FDC 30 has failed during travel of the fuel cellelectric vehicle and this failure is the voltage boosting-disablingfailure in which the FDC 30 cannot perform the voltage boosting actionbut can pass a current, the voltage boosting action of the FDC 30 isstopped and a current is passed through the FDC 30. Thus, electricitycan be continuously supplied from the fuel cell 20 to the load 70, sothat shortening of the distance that the fuel cell electric vehicle cantravel can be prevented compared with if electricity is supplied onlyfrom the secondary battery 40.

When the voltage boosting-disabling failure is a failure of the firstvoltage sensor 37, the detection value of the second voltage sensor 25is used as the voltage value on the secondary side of the FDC 30. Thus,the output electricity of the fuel cell system 10 can be controlled. Thedetection value of the second voltage sensor 25 that is theoreticallyequal to the detection value of the first voltage sensor 37 is used asthe voltage value on the secondary side of the FDC 30. Therefore, anerror from the actual voltage value can be avoided.

When the voltage boosting-disabling, failure is a failure of the firstcurrent sensor 36, the current value of the FDC 30 is estimated by usingthe detection value of the second current sensor 41 and the totalelectricity supplied by the fuel cell system 10, and the estimatedcurrent value is used as the current value of the FDC 30. Thus, theoutput electricity of the fuel cell system 10 can be continuouslycontrolled. Moreover, compared with if the current value of the FDC 30is estimated by using an I-V curve that indicates I-V characteristics ofthe fuel cell 20, the influence of fluctuations in the I-V curve due todeterioration etc. of the fuel cell 20 can be avoided, and thus an errorfrom the actual current value can be avoided.

In the voltage boosting-less mode, electricity can be continuouslysupplied not only from the fuel cell 20 but also from the secondarybattery 40. Thus, when the voltage required by the load 70 is higherthan the voltage on the secondary side of the FDC 30, a voltagecorresponding to the shortage can be supplied from the secondary battery40. Therefore, the supply voltage can be prevented from falling short ofthe voltage required by the load 70.

In the case of a failure in which the FDC 30 can perform the voltageboosting action, the FDC 30 performs the voltage boosting action byusing the IPMs 31 of phases that are not having a failure. Thus,electricity can be continuously supplied from the fuel cell 20 to theload 70 to prevent shortening of the distance that the fuel cellelectric vehicle can travel.

B. Other Embodiments B-1. Second Embodiment

In the failure-response control process of the above embodiment, thevoltage value to be used as the output voltage value of the FDC 30 isswitched to the voltage value detected by the second voltage sensor 25in step S254. However, the present disclosure is not limited to thisexample. For example, the voltage value to be used as the output voltagevalue of the FDC 30 may be switched to the voltage value detected by thethird voltage sensor 22 instead of the voltage value detected by thesecond voltage sensor 25. This measure takes advantage of the fact thatthe voltage on the primary side and the voltage on the secondary side ofthe FDC 30 become theoretically equal when the voltage boosting actionof the FDC 30 is stopped. In this aspect, the third voltage sensor 22can be regarded as a subordinate concept of the fuel cell voltage sensormentioned in the SUMMARY. In another aspect, for example, the voltagevalue to be used as the output voltage value of the FDC 30 may beswitched to a value obtained by adding up the values of the cellvoltages of the respective single cells of the fuel cell 20 detected bythe cell monitor 21. In this aspect, the cell monitor 21 can be regardedas a subordinate concept of the fuel cell voltage sensor mentioned inthe SUMMARY. For example, in these aspects in which the voltage value isswitched as described above, a value corrected based on a difference ofsensors between the first voltage sensor 37 on one hand and the secondvoltage sensor 25, the third voltage sensor 22, and the cell monitor 21on the other hand, may be used as the output voltage value of the FDC30. For example, the output voltage value of the FDC 30 may be estimatedbased on the current value of the FDC 30 and the I-V curve indicatingthe I-V characteristics of the fuel cell 20 that is stored in advance inthe storage device of the control unit 90, and the voltage value to beused as the output voltage value of the FDC 30 may be switched to thisestimation value. These configurations can achieve effects similar tothose of the above embodiment.

B-2. Third Embodiment

In the failure-response control process of the above embodiment, thecurrent value of the FDC 30 is estimated by using the total electricitysupplied by the fuel cell system 10 and the current value of thesecondary battery 40 detected by the second current sensor 41 in stepS258. However, the present disclosure is not limited to this example.For example, the current value of the FDC 30 may be estimated from thesum of the detection values of the third current sensors 39 of the IPMs31. Alternatively, for example, the current value of the FDC 30 may beestimated based on the output voltage value of the FDC 30 and the I-Vcurve indicating the I-V characteristics of the fuel cell 20 that isstored in advance in the storage device of the control unit 90. Theseconfigurations can achieve effects similar to those of the aboveembodiment.

B-3. Fourth Embodiment

The contents of the control in the voltage boosting-less mode in theabove embodiment is merely an example and can be modified in variousways. For example, in the voltage boosting-less mode, the upper arm 56of the BDC 50 may be turned on and the lower arm 58 of the BDC 50 may beturned off, to thereby stop the voltage boosting action of the BDC 50and directly connect the primary-side line 53 and the secondary-sideline 54 to each other. In this configuration, electricity to be consumedin the load 70 is supplied preferentially from one of the fuel cell 20and the secondary battery 40 that has a higher voltage, and electricityis supplied from both the fuel cell 20 and the secondary battery 40 froma point when the voltage of the fuel cell 20 and the voltage of thesecondary battery 40 become equal. By thus stopping the voltage boostingaction of the BDC 50, it is possible to avoid a situation where theoutput voltage of the BDC 50 becomes higher than the output voltage ofthe FDC 30 and electricity is supplied only from the secondary battery40. Thus, over-discharge of the secondary battery 40 can be avoided.Moreover, an amount of electricity equal to or larger than a ratedamount of dischargeable electricity of the secondary battery 40 can beprevented from being supplied from the secondary battery 40, so thatdeterioration of the secondary battery 40 can be avoided.

In the voltage boosting-less mode, stopping the voltage boosting actionof the FDC 30 makes it difficult to control the amount of electricitygenerated by the fuel cell 20. Therefore, for example, an extra amountof fuel gas may be supplied to the fuel cell 20 and thus the amount ofelectricity generated may be increased compared with under normalcontrol when the FDC 30 is having no failure, in order to obtainelectricity required by the load 70. When extra electricity equal to orhigher than required electricity is generated by such electricitygeneration, the upper arm of the BDC 50 may be turned on to charge thesecondary battery 40.

For example, in the voltage boosting-less mode, the voltage of the fuelcell 20 may be maintained at a higher value than the voltage of thesecondary battery 40. More specifically, the cell voltage may bemaintained at a higher potential and the SOC of the secondary battery 40may be lowered to lower the voltage of the secondary battery comparedwith under normal control when the FOG 30 is having no failure. The SOCof the secondary battery 40 can be lowered by adjusting the timing atwhich the upper arm 56 of the BDC 50 is turned on. When the voltage ofthe fuel cell 20 is maintained at a higher value than the voltage of thesecondary battery 40, electricity can be supplied preferentially fromthe fuel cell 20 to the load 70. Thus, it is possible to continue theelectricity generation operation of the fuel cell 20 while avoiding asituation where electricity is supplied preferentially from thesecondary battery 40 to the load 70 and the fuel cell 20 generates extraelectricity. Since the voltage on the primary side of the BDC 50 islower than the voltage on the secondary side of the FDC 30, a margin ofvoltage boosting by the BDC 50 can be secured. To avoid making thevoltage boosting action unstable, such a margin of voltage boosting issecured that the voltage on the primary side of the BDC 50 may beboosted to a value equal to the voltage on the secondary side of the FDC30 at a voltage boosting ratio equal to or higher than a predeterminedminimum voltage boosting ratio. Thus, it is possible to avoid asituation where the voltage boosting action of the BDC 50 becomesunstable and the accuracy of voltage boosting decreases, as well as asituation where the voltage boosting action of the BDC 50 stops and acurrent passing through the BDC 50 cannot be controlled.

As electricity to be supplied to the auxiliaries 80 in the voltageboosting-less mode, for example, the electricity supplied from the fuelcell 20 as the upper arm of the BDC 50 is turned on may also be used inaddition to the electricity supplied from the secondary battery 40. Forexample, to reduce electricity supplied by the fuel cell system 10, theelectricity supplied to the auxiliaries 80 may be minimized. Forexample, the coolant heater 84, the air-conditioning device 85, etc. maybe stopped. These configurations can achieve effects similar to theabove embodiment.

B-4. Fifth Embodiment

The failure-response control process in the above embodiment is merelyan example and can be modified in various ways. For example when it isdetermined in step S230 that the failure is not a failure in which theFDC 30 cannot perform the voltage boosting action (step S230: NO), thecontrol unit 90 may move to step S250 and shifts the operation mode tothe voltage boosting-less mode. For example, in step S270, at least oneof the contents of the control in the secondary battery running mode,i.e., stopping the FDC 30, stopping electricity generation of the fuelcell 20, and opening the fuel cell relay circuit 29, may be omitted.These configurations can achieve effects similar to those of the aboveembodiment.

B-5. Sixth Embodiment

In the above embodiment, the FDC 30 is configured as a four-phase bridgeconverter, but the present disclosure is not limited to this example.The number of phases of the FDC 30 may be one, two, three, or five orlarger, instead of four. These configurations can achieve effectssimilar to those of the above embodiment.

B-6. Seventh Embodiment

In the above embodiment, the fuel cell system 10 has been described asincluding the secondary battery 40 and the BDC 50, but the presentdisclosure is not limited to this example. The secondary battery 40 andthe BDC 50 may not be components of the fuel cell system 10, and may beable to supply electricity to the load 70 by being connected to the load70, separately from the fuel cell system 10. In the above embodiment,the FDC 30 has been described as having the first voltage sensor 37 andthe first current sensor 36, but the present disclosure is not limitedto this example. The first voltage sensor 37 and the first currentsensor 36 may not be components of the FDC 30, and may be electricallyconnected to the FDC 30 so as to be able to detect the voltage value onthe secondary side of the FDC 30 and the current value of the FDC 30,respectively. These configurations can achieve effects similar to thoseof the above embodiment.

B-7. Eighth Embodiment

The control unit 90 of the above embodiment is a single ECU but mayinstead be composed of a plurality of ECUs. When the control unit 90 iscomposed of a plurality of ECUs, control over electricity generation ofthe fuel cell 20, control over the voltage boosting actions of the FDC30 and the BDC 50, etc. may be performed by different ECUs. The controlunit 90 may be formed as a part of a unit that controls the fuel cellelectric vehicle. These configurations can achieve effects similar tothose of the above embodiment.

B-8. Ninth Embodiment

In the above embodiment, the fuel cell system 10 is used by beinginstalled in the fuel cell electric vehicle. However, the fuel cellsystem 10 may be installed in another arbitrary mobile object, such as aship or a robot, instead of a vehicle. These configurations can achieveeffects similar to those of the above embodiment.

The present disclosure is not limited to the above embodiments but canbe implemented in various configurations without departing from the gistof the disclosure. For example, the technical features of theembodiments corresponding to the technical features of the aspectsdescribed in the SUMMARY can be replaced or combined as appropriate inorder to solve some or all of the above problems or to achieve some orall of the above effects. Unless a technical feature is described asbeing essential in the specification, the technical feature can beomitted as appropriate.

What is claimed is:
 1. A fuel cell system comprising: a fuel cell thatsupplies electricity to a load; a fuel cell converter that is connectedbetween the fuel cell and the load and boosts a voltage output from thefuel cell; and a control unit programmed to cause the fuel cellconverter to perform a voltage boosting action, and programmed tocontrol output electricity to the load, the control unit is programmedto, upon detecting a voltage boosting-disabling failure that is afailure in which the fuel cell converter is unable to perform thevoltage boosting action and able to pass a current, stop the voltageboosting action of the fuel cell converter and pass a current generatedby the fuel cell through the fuel cell converter.
 2. The fuel cellsystem according to claim 1, further comprising: a secondary batterythat supplies electricity to the load; a secondary battery converterthat is connected between the secondary battery and the load, has anoutput terminal electrically connected to an output terminal of the fuelcell converter, and converts an output voltage of the secondary battery;a first voltage sensor that detects a voltage value on a secondary sideof the fuel cell converter; and a second voltage sensor that detects avoltage value on a secondary side of the secondary battery converter,wherein the voltage boosting-disabling failure includes a failure of thefirst voltage sensor, and the control unit is programmed to, when thevoltage boosting-disabling failure is the failure of the first voltagesensor, control the output electricity using a detection value of thesecond voltage sensor as the voltage value on the secondary side of thefuel cell converter.
 3. The fuel cell system according to claim 1,further comprising: a first voltage sensor that detects a voltage valueon a secondary side of the fuel cell converter; and a fuel cell voltagesensor that detects a voltage value of the fuel cell, wherein thevoltage boosting-disabling failure includes a failure of the firstvoltage sensor, and the control unit is programmed to, when the voltageboosting-disabling failure is the failure of the first voltage sensor,control the output electricity using a detection value of the fuel cellvoltage sensor as the voltage value on the secondary side of the fuelcell converter.
 4. The fuel cell system according to claim 1, furthercomprising: a secondary battery that supplies electricity to the load; afirst current sensor that detects a current value of the fuel cellconverter; and a second current sensor that detects a current value ofthe secondary battery, wherein the voltage boosting-disabling failureincludes a failure of the first current sensor, and the control unit isprogrammed to, when the voltage boosting-disabling failure is thefailure of the first current sensor, estimate the current value of thefuel cell converter by using a detection value of the second currentsensor and total electricity supplied by the fuel cell system, andcontrol the output electricity using the estimated current value as thecurrent value of the fuel cell converter.
 5. A control method of a fuelcell system having a fuel cell converter that is connected between afuel cell and a load and boosts an output voltage of the fuel cell and acontrol unit programmed to execute the control method, the controlmethod comprising, upon detecting a voltage boosting-disabling failurethat is a failure in which the fuel cell converter is unable to performa voltage boosting action and able to pass a current, stopping thevoltage boosting action of the fuel cell converter and passing a currentgenerated by the fuel cell through the fuel cell converter.